Development of deep hypothermia in rats with limited motor activity

Changes of the main organism functions (breathing frequency, heart rate and shivering) were investigated under hypothermia in two groups of rats. Animals of the first group were fixed rigidly on the special platform with fixing of head and limbs, and those of the second one--the rats, were placed in a punched cylindrical chamber, inside which they could move freely forward and back. In 2.5-3.0 hours after anaesthesia the rats were placed in a refrigerator (-5 degrees C) until they stop breathing. Cessation of breathing of the first group rats occurred in 1.7 +/- 0.3 hours from the beginning of cooling at body temperature 17.3 +/- 0.6 degrees C and the brain temperature 15.7 +/- 0.5 degrees C. In the second group, a prolonged activation of the frequency of breathing, heart rate and intensity of electrical activity of muscles during 2.5-3.0 hours, was observed. Only in 4.5-5.0 hours, the breathing stopped at rectal temperature 12.3 +/- 1.1 degrees C and the brain temperature 12.9 +/- 0.9 degrees C. In these animals, the time of survival in the cold environment increased considerably and the temperature thresholds of the termination of breathing were lowered. Thus, the activation in the thermo-regulative muscle tone and in shivering muscles provides the most effective resistance against cooling of rats, reducing a surface of heat, dissipation and keeping the temperature of internal areas of body.     

The physiology of hypothermia in the black-capped Chickadee,Parus atricapillus

Summary The shivering, body temperature, and metabolic response to stable and decreasing ambient temperature were measured in winter acclimatized Black-capped Chickadees,Parus atricapillus. Shivering activity, measured by duration and amplitude of bursts, increased curvilinearly from thermoneutral temperatures of 27°C down to 0°C. This parabolic shivering response may be a major component of the curvilinear response of metabolism to decreasing ambient temperature.Birds exposed to 0°C exhibited metabolism 32–45% lower than predicted for a 12-g homeotherm and body temperatures 10°C below the pre-experimental nocturnal body temperature. This hypothermia was not the result of a breakdown in thermoregulation, but was a controlled effort serving to reduce overnight energy expenditure. It is suggested that (1) hypothermia was achieved by decreased shivering by pectoral muscles during exposure to decreasing ambient temperatures, (2) the rate of body temperature decline was moderated by intermittent and reduced bursts during the cooling period, and (3) body temperature was maintained at a particular level during exposure to a stable low ambient temperature by intense bursts lasting one to three minutes.The physiology of hypothermia in chickadees is similar to torpor; however, chickadees did not arouse to a normal diurnal body temperature in the laboratory, and their hypothermia was not induced by inanition or prolonged exposure to cold, as reported for other species capable of torpor.     

The effect of benzodiazepine on the increase of oxygen consumption in cold exposed rats

1. Oxygen consumption and rectal temperature of warm and cold acclimated rats were measured after chronic and acute injections of saline or benzodiazepine (diazepam). 2. Benzodiazepine has blocked the increase in oxygen consumption of warm acclimated rats on exposure to cold. 3. After cold acclimation, the benzodiazepine did not affect the increase in oxygen consumption. 4. Benzodiazepine caused a slight hypothermia when injected chronically, but did not affect rectal temperature over a short period of time.     

Does cold acclimation induce nonshivering thermogenesis in juvenile birds? Experiments with Pekin ducklings and Japanese quail chicks

The capability to produce heat in cold by nonshivering thermogenesis (NST) was studied in Pekin ducklings and Japanese quail chicks acclimated to cold for 3 weeks using indirect calorimetry (oxygen consumption) and electromyography from breast (M. pectoralis) and leg muscles (quails: M. gastrocnemius; ducklings: M. gastrocnemius, M. iliofibularis). Respiration of muscles in vitro was studied by measuring cytochrome c oxidase activity. In both species, cold acclimation induced clear morphometric and physiological changes, but no clear evidence of nonshivering thermogenesis. This was evident because increased shivering at least in one muscle coincided with increased oxygen consumption. In ducklings, however, amplitudes of shivering EMGs were low (<30 μV) in all muscles studied in both the control and cold-acclimated groups. Ducklings reacted to cold mainly by means of increasing body weight (1796 g in control, 2095 g in cold-acclimated) and circulatory changes. Acclimation did not change oxygen consumption either in vivo or in vitro. In quails, in addition to increased body weight (78.1 g control, 89.9 g cold-acclimated), improved insulation and metabolic adaptation to cold (increased respiration in vivo and in M. pectoralis in vitro) was also utilized. In Japanese quail chicks, 3 weeks of cold acclimation does not seem to induce NST, while in Pekin ducklings the existence of NST could not be totally excluded because of weak overall shivering activity. Accepted: 13 July 2000     

Inhibition of shivering during restraint hypothermia

Restraint hypothermia has often been described, but its cause has never been clarified. We hypothesized that it might be due to a suppression of shivering thermogenesis. Thus, we restrained conscious rats in an ambient temperature of 2 degrees C while measuring rectal (Tre) and tail skin temperatures, metabolic rate (MR), and shivering activity. When rats were cold exposed but not restrained, Tre fell 1.4 +/- 0.2 degrees C (SE) during the 1st h. When these same rats were restrained, Tre fell at a rate of 6.5 +/- 0.2 degrees C/h. MR averaged 15.7 +/- 1.4 W/kg for the unrestrained rats, but it averaged only 9.0 +/- 1.1 W/kg for the restrained rats. The restrained rats showed no signs of shivering. The animals were then subjected to a restraint adaptation regimen and then reexposed to cold. Restraint now produced a fall in Tre of only 2.6 +/- 0.7 degrees C/h. The animals shivered and generated an MR of 15.8 +/- 0.9 W/kg. Naive rats became hypothermic because restraint suppressed shivering activity. However, adapted rats continued to shiver and remained normothermic. We suggest that a stressful or threatening situation, such as restraint for a naive rat, inhibits shivering and leads to hypothermia in a cold environment. This would not occur in adapted rats because restraint is no longer stressful.     

Relative intensity of muscular contraction during shivering.

The intensity of cold-induced shivering, quantified by surface electromyography (EMG) and then expressed as a function of the maximal myoelectrical activity (integrated EMG) obtained during a maximum voluntary contraction (MVC), was examined in this study in individuals classified by body fat. In addition, the relationship between shivering and metabolic rate (MR) and the relative contribution of various muscle groups to total heat production were studied. Ten seminude male volunteers, 5 LEAN (less than 11% body fat) and 5 NORM (greater than 15% body fat) were exposed to 10 degrees C air for 2 h. EMG of six muscle groups (pectoralis major, rectus abdominis, rectus femoris, gastrocnemius, biceps brachii, and brachioradialis) was measured and compared with the EMG of each muscle's MVC. A whole body index of shivering, determined from the mass-weighted intensity of shivering of each muscle group, was correlated with MR. After the initial few minutes of exposure, only the pectoralis major, rectus femoris, and biceps brachii continued to increase their intensity of shivering. Shivering intensity was higher in the central muscles, ranging from 5 to 16% of MVC compared with that in the peripheral muscles, which ranged from 1 to 4% of MVC. Shivering intensities were similar in the peripheral muscles for the LEAN and NORM groups, whereas differences occurred in the trunk muscles for the pectoralis major and rectus abdominis. The whole body index of shivering correlated significantly with each individual's increase in MR (r = 0.63-0.97).(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of carbohydrate availability on sustained shivering II. Relating muscle recruitment to fuel selection

The purpose of this study was to quantify how shivering activity would be affected by large changes in fuel metabolism (see Haman F, Peronnet F, Kenny GP, Doucet E, Massicotte D, Lavoie C, and Weber J-M, J Appl Physiol 96: 000-000, 2004). Adult men were exposed to 10 degrees C for 2 h after a low-carbohydrate diet and exercise (Lo) and after high-carbohydrate diet without exercise (Hi). Using simultaneous metabolic and electromyographic (EMG) measurements, we quantified the effects of changes in fuel selection on the shivering activity of eight large muscles representing >90% of total shivering muscle mass. Contrary to expectation, drastic changes in fuel metabolism [carbohydrates 28 vs. 65% of total heat production (Hprod), lipids 53 vs. 23% Hprod, and proteins 19 vs. 12% Hprod for Lo and Hi, respectively] are achieved without altering the EMG signature of shivering muscles. Results show that total shivering activity and the specific contribution of each muscle to total shivering activity are not affected by large changes in fuel selection. In addition, we found that changes in burst shivering rate ( approximately 4 bursts/min), relative contribution of burst activity to total shivering ( approximately 10% of total shivering activity), and burst shivering intensity ( approximately 12% of maximal voluntary contraction) are the same between Lo and Hi. Spectral analysis of EMG signals also reveals that mean frequencies of the power spectrum remained the same under all conditions (whole body average of 78 +/- 5 Hz for Lo and 83 +/- 7 Hz for Hi). During low-intensity shivering, humans are therefore able to sustain the same thermogenic rate by oxidizing widely different fuel mixtures within the same muscle fibers.     

Synchronized slow-amplitude modulations in the electromyograms of shivering muscles

The electromyograms (EMG) of shivering human subjects exposed to 0 degrees C air in an environmental chamber were analyzed to detect slow-amplitude modulations (SAMs, less than 1 Hz) in the EMGs of widely separated muscles and to study the relationship of these SAMs to respiration rate and skin temperature. Distinct amplitude modulations were observed in the raw EMGs during shivering. The peaks in EMG activity occurred simultaneously in the majority of the monitored muscles in all subjects. Pearson correlations between the average rectified EMGs of 93% of the muscles were significant (P less than 0.05). Visual analysis of the EMG and respiration signals indicated that the peaks in muscular activity occurred 6-12 times/min, whereas respiration ranged from 10 to 23 cycles/min. For all subjects respiration was at a higher frequency than amplitude modulation in the EMG. Comparison of EMG records with expiratory flow rate traces in shivering subjects indicated no one-to-one correlation between the occurrence of respiration and EMG amplitude modulations. Respiratory flow rate and average rectified EMG showed significant correlation in only 33% of the cases. In addition, skin temperature changes could not be correlated with the SAMS.     

Response to cold in the blue fox and raccoon dog as evaluated by metabolism, heart rate and muscular shivering: a re-evaluation

Oxygen consumption (ml kg-0.75/min) in relation to ambient air temperature at or below the lower critical temperature (Tlc) of the winter-furred raccoon dog (+10 degrees C) and the blue fox (-6 degrees C) is described by the equations y = 14.8-0.28x and y = 7.5-0.20x, respectively. Muscular shivering activity (integrated EMG) of both species increased below thermoneutrality parallel with increasing oxygen uptake and heart rate. Seasonal changes in measured metabolic parameters were evident for both species. The results suggest that the overall body insulation or the metabolic response to cold are not essentially worse in the raccoon dog as compared with the blue fox. It is concluded that earlier speculations of surprisingly wide thermoneutral zone and very low Tlc of the Arctic fox are not evident for the blue fox.     

Threshold for shivering in aerobic and anaerobic muscles in bantam cocks and incubating hens

Summary Electromyographic activity (EMG) from the musculus pectoralis (breast muscle), m. iliotibialis (thigh muscle) and m. gastrocnemius (leg muscle), cloacal temperature (Tb) and O₂ consumption were measured in bantam cocks (Gallus domesticus) exposed to different ambient temperatures (Ta). The same parameters were measured in bantam hens incubating eggs artificially thermoregulated to 40° and 25°C (Te).EMG activity appeared in thigh and leg muscles at Ta below 32°C (Tsh). This temperature probably represents the thermoneutral temperature (TNT) of the cock. EMG activity in breast muscles appeared at Ta below 20°C, or 4°C below the lower critical temperature (Tc).All muscles were quiet when the hen incubated 40°C egg at Ta=Tsh. When Te was abruptly changed to 25°C, EMG activity in the iliotibialis muscle appared 3 min before the activity in the pectoralis muscle. Tb dropped from 41.2° to 40.6°C in 14 min. When Te was returned to 40°C, the EMG activity in the pectoralis muscle disappeared almost at once, while the iliotibialis muscle was active until Tb returned to normal.Aerobic muscles seem to be responsible for shivering thermogenesis between Tc and Tsh, while anaerobic muscles are recruited at lower Ta or when the heat loss during incubation becomes severe.Abbreviations EMG electromyography - Ta ambient temperature - Tb cloacal temperature - Tc lower critical temperature - Te egg temperature - TNT thermoneutral temperature - Tsh shivering threshold temperature     

Mechanisms of suppression of physiological function in hypothermia and a way of their restoration without body warming

It was shown that in hypothermic rats (rectal temperature 25-22 degrees C) it was possible to stimulate responses that had been suppressed by cold (i. e. thermoregulation and breathing) with the aid of injecting a solution of ethylenediaminetetraacetic acid disodium salt (EDTA) in quantity 16.5 mg/100 g of body weight (0.0045 mmol/100 g) into the blood stream of the cooled animals. EDTA connects calcium ions in blood and forms complexes. It was shown that enhancement of cold shivering intensity and that of breathing (in 5 min after beginning the injection of EDTA) coincided with a 42-45 % reduction of [Ca2+] in the blood]. After 15 min following the beginning of injection of EDTA [Ca2+] into the blood stream, a return to the initial level was observed in cooled animals. Simultaneously we observed suppression of the cold shivering and breathing. The repeated injection of EDTA again caused similar fall of [Ca2+] in the blood and the following enhancement of cold shivering and breathing.     

Brünnich's guillemots (Uria lomvia) maintain high temperature in the body core during dives

A major challenge for diving birds, reptiles, and mammals is regulating body temperature while conserving oxygen through a reduction in metabolic processes. To gain insight into how these needs are met, we measured dive depth and body temperatures at the core or periphery between the skin and abdominal muscles simultaneously in freely diving Brünnich's guillemots (Uria lomvia), an arctic seabird, using an implantable data logger (16-mm diameter, 50-mm length, 14-g mass, Little Leonardo Ltd., Tokyo). Guillemots exhibited increased body core temperatures, but decreased peripheral temperatures, during diving. Heat conservation within the body core appeared to result from the combined effect of peripheral vasoconstriction and a high wing beat frequency that generates heat. Conversely, the observed tissue hypothermia in the periphery should reduce metabolic processes as well as heat loss to the water. These physiological effects are likely one of the key physiological adaptations that makes guillemots to perform as an efficient predator in arctic waters.     

Daily rhythm of oxygen consumption and thermoregulatory responses in some European winter- or summer-acclimatized finches at different ambient temperatures

The oxygen consumption of European finches, the siskin (Carduelis spinus), the brambling (Fringilla montifringilla), the bullfinch (Pyrhulla pyrhulla), the greenfinch (Carduelis chloris) and the hawfinch (Coccothraustes coccothraustes), was recorded continuously while ambient temperature was decreased stepwise from +30 down to-75°C. The oxygen consumption, body temperature (telemetrically), and shivering (integrated pectoral electromyography) of greenfinches were measured simultaneously at ambient temperatures between +30 and-75°C. Maximum heat production, cold limit, lower critical temperature, basal metabolic rate and thermal conductance (of the greenfinch) were determined. The diurnal variation of oxygen consumption of siskins and greenfinches was recorded at thermoneutrality and below the thermoneutral zone in winter- and summer-acclimatized birds. The diurnal variation of body temperature and thermal conductance of greenfinches were also determined. The diurnal variation of heat production was not seasonal or temperature dependent in the siskin and in the greenfinch. Nocturnal reduction of oxygen consumption saved 15–33% energy in the siskin and greenfinch. Body temperature of the greenfinch was lowered by 2.5–3.4°C. The nocturnal reduction of thermal conductance in the greenfinch was 39–48%. The basal metabolic rate was lowest in the largest bird (hawfinch) and highest in the smallest bird (siskin). The values were in the expected range. The heat production capacity of finches in winter was 4.7 times basal metabolic rate in the siskin, 4.2 times in the brambling, 3.5 times in the greenfinch and 2.9 times in the bullfinch and hawfinch. The heat production capacity of the siskin and greenfinch was not significantly lower in summer. The cold limit temperatures (°C) in winter were-61.2 in the siskin,-41.3 in the greenfinch,-37.0 in the bullfinch,-35.7 in the brambling and-28.9 in the hawfinch. The cold limit was 14.3°C higher in summer than in winter in the siskin and 8.7°C in the greenfinch. Thermal insulation of the greenfinch was significantly better in winter than in summer. The shivering of the greenfinch increased linearly when ambient temperature was decreased down to-40°C. Maintenance of shivering was coincident with season. In severe cold integrated pectoral electromyography did not correlate with oxygen consumption as expected. The possible existence of non-shivering thermogenesis in birds is discussed. It is concluded that the acclimatization of European finches is primarily metabolic and only secondly affected by insulation.Abbreviations AAT avian adipose tissue - bm body mass - BMR basal metabolic rate - C _t thermal conductance - EMG electromyogram - HP heat production - HP _max maximum heat production - MR metabolic rate - NST non-shivering thermogenesis - RMR resting metabolic rate - RQ respiratory quotient - T _a ambient temperature - T _b body temperature - T _c colonic temperature - T _1c lower critical temperature - TNZ thermoneutral zone - T _st shivering threshold temperature - V oxygen consumption     

Shivering onset, metabolic response, and convective heat transfer during cold air exposure

The onset and intensity of shivering of various muscles during cold air exposure are quantified and related to increases in metabolic rate and convective heat loss. Thirteen male subjects resting in a supine position and wearing only shorts were exposed to 10 degrees C air (42% relative humidity and less than 0.4 m/s airflow) for 2 h. Measurements included surface electromyogram recordings at six muscle sites representing the trunk and limb regions of one side of the body, temperatures and heat fluxes at the same contralateral sites, and metabolic rate. The subjects were grouped according to lean (LEAN, n = 6) and average body fat (NORM, n = 7) content. While the rectal temperatures fluctuated slightly but not significantly during exposure, the skin temperature decreased greatly, more at the limb sites than at the trunk sites. Muscles of the trunk region began to shiver sooner and at a higher intensity than those of the limbs. The intensity of shivering and its increase over time of exposure were consistent with the increase in the convective heat transfer coefficient calculated from skin temperatures and heat fluxes. Both the onset of shivering and the magnitude of the increase in metabolic rate due to shivering were higher for the LEAN group than for the NORM group. A regression analysis indicates that, for a given decrease in mean skin temperature, the increase in metabolic rate due to shivering is attenuated by the square root of percent body fat. Thus the LEAN group shivered at higher intensity, resulting in higher increases in metabolic heat production and convective heat loss during cold air exposure than did the NORM group.     

High aerobic capacities in the skeletal muscles of pinnipeds: adaptations to diving hypoxia.

The objective was to assess the aerobic capacity of skeletal muscles in pinnipeds. Samples of swimming and nonswimming muscles were collected from Steller sea lions (Eumetopias jubatus, n = 27), Northern fur seals (Callorhinus ursinus, n = 5), and harbor seals (Phoca vitulina, n = 37) by using a needle biopsy technique. Samples were either immediately fixed in 2% glutaraldehyde or frozen in liquid nitrogen. The volume density of mitochondria, myoglobin concentration, citrate synthase activity, and beta-hydroxyacyl-CoA dehydrogenase was determined for all samples. The swimming muscles of seals had an average total mitochondrial volume density per volume of fiber of 9.7%. The swimming muscles of sea lions and fur seals had average mitochondrial volume densities of 6.2 and 8.8%, respectively. These values were 1.7- to 2.0-fold greater than in the nonswimming muscles. Myoglobin concentration, citrate synthase activity, and beta-hydroxyacyl-CoA dehydrogenase were 1.1- to 2. 3-fold greater in the swimming vs. nonswimming muscles. The swimming muscles of pinnipeds appear to be adapted for aerobic lipid metabolism under the hypoxic conditions that occur during diving.     

Effects of the menstrual cycle on muscle recruitment and oxidative fuel selection during cold exposure

Differences in core temperature and body heat content, generally observed between the luteal and follicular phase of the menstrual cycle, have been reported to modulate the thermogenic activity of cold-exposed women. However, it is unclear how this change in whole body shivering activity will influence fuel selection. The goal of this study was to quantify the effects of the menstrual cycle on muscle recruitment and oxidative fuel selection during low-intensity shivering. Electromyographic activity of eight large muscles was monitored while carbohydrate, lipid, and protein utilization was simultaneously quantified in the follicular and luteal phases of the menstrual cycle in nonacclimatized women shivering at a low intensity. The onset (～25 min), intensity (～15% of maximal voluntary contraction), and pattern (～6 shivering bursts/min) of the shivering response did not differ between menstrual cycle phases, regardless of differences in core temperature and hormone levels. This resulted in lipids remaining the predominant substrate, contributing 75% of total heat production, independent of menstrual phase. We conclude that hormone fluctuations inherent in the menstrual cycle do not affect mechanisms of substrate utilization in the cold. Whether the large contribution of lipids to total heat production in fuel selection confers a survival advantage remains to be established.     

Fever in young lambs: temperature, metabolic and cardiorespiratory responses to a small dose of bacterial pyrogen

Experiments were done on ten lambs ranging in age from 15 to 25 days to define the temperature, metabolic and cardiorespiratory responses to intravenous administration of a small dose of bacterial pyrogen (SAE). Administration of SAE but not normal saline produced a short-lived fever of about 0.7 degrees C. The increase in body-core temperature was preceded by a surge in total body oxygen consumption and the onset of shivering which was influenced by behavioral state (ie, shivering was inhibited during active sleep). The increase in total body oxygen consumption was initially met by an increase in total body oxygen extraction and then by an increase in systemic oxygen delivery. Systemic arterial blood pressure did not change significantly during the febrile response; however, pulmonic arterial blood pressure increased significantly. Thus, our experiments provide new data on oxygen supply and demand during the development of fever and that shivering thermogenesis is inhibited in active sleep following the administration of bacterial pyrogen in young lambs. The influence of active sleep on the overall febrile response, and whether or not there is a shift from shivering thermogenesis to non-shivering thermogenesis remains to be determined.     

Aerobic metabolism of American alligators, Alligator mississippiensis, under standard conditions and during voluntary activity

We measured the rate of consumption of oxygen by alligators in a dry metabolic chamber and in a tank of water where they were free to dive and surface at will at 10-35 degrees C, a range spanning most of the body temperatures experienced by alligators in nature. Neither the standard metabolic rate nor the rate of oxygen consumption during one hour of sustained, voluntary activity varied with body mass, month of the year, duration of fasting before measurement, or experimental condition (terrestrial vs aquatic). Voluntary diving is not accompanied by any reduction in standard metabolic rate; these results and those of others suggest that the "diving reflex" of alligators is probably employed only in emergencies. Spontaneous activity for one hour is accompanied by a 1.9-4.4 fold rise in oxygen consumption; this factorial increase is less than that for other reptiles induced to maximal activity for brief intervals. Both standard and active oxygen consumption rise significantly with body temperature.     

Effect of photoperiod and melatonin on cold resistance,thermoregulation and shivering/nonshivering thermogenesis in Japanese quail

Summary The effect of photoperiod and melatonin treatment on cold resistance and thermogenesis of quails was studied. The birds were acclimated for 8 weeks to short day (8L:16D) or long day (16L:8D) conditions, and 8 of 16 quails in each group were implanted with melatonin capsules. One group of quails was maintained outside in an aviary during winter. Oxygen consumption ( ) body temperature (T _b, recorded with temperature transmitters) and shivering (integrated pectoral EMG) were recorded continuously, and samples of heart rate and breathing rate were picked up when ambient temperature was decreased stepwise from 27 down to –75 °C. Heat production maximum (HP_max), cold limit, lower critical temperature, basal metabolic rate (BMR) and thermal conductance were determined.The results show that short day, cold and melatonin treatment improved cold resistance and thermal insulation of quils when compared with quails acclimated to long day conditions. An increase in HP_max was induced only by melatonin treatment. The results suggest that the acclimatization of quails is under control of the pineal gland.The linear increase of shivering intensity with at moderate cold load shows that shivering is the primary source for thermoregulatory heat production in the quail. AtT _a's below –40 °C shivering remained constant although , heart rate and breathing rate continued to increase with increasing cold load. This could indicate the existence of a nonshivering thermogenesis in birds. Unlike to mammals, this non-shivering thermogenesis in birds would serve as secondary source of heat supporting shivering thermogenesis in severe coldAbbreviations BMR basal metabolic rate - ECG electrocardiogram - EMG electromyogram - NST nonshivering thermogenesis - SMR standard metabolic rate     

Human thermoregulatory responses during prolonged walking in water at 25, 30 and 35°C

Eight healthy and physically well-trained male students exercised on a treadmill for 60 min while being immersed in water to the middle of the chest in a laboratory flowmill. The water velocity was adjusted so that the intensity of exercise correspond to 50% maximal oxygen uptake of each subject, and experiments were performed once at each of three water temperatures: 25, 30, 35°C, following a 30-min control period in air at 25°C, and on a treadmill in air at an ambient temperature of 25°C. Thermal states during rest and exercise were determined by measuring rectal and skin temperatures at various points, and mean skin temperatures were calculated. The intensity of exercise was monitored by measuring oxygen consumption, and heart rate was monitored as an indicator for cardiovascular function. At each water temperature, identical oxygen consumption levels were attained during exercise, indicating that no extra heat was produced by shivering at the lowest water temperature. The slight rise in rectal temperature during exercise was not influenced by the water temperature. The temperatures of skin exposed to air rose slightly during exercise at 25°C and 30°C water temperature and markedly at 35°C. The loss of body mass increased with water temperature indicating that both skin blood flow and sweating during exercise increased with the rise in water temperature. The rise in body temperature provided the thermoregulatory drive for the loss of the heat generated during exercise. Heart rate increased most during exercise in water at 35°C, most likely due to enhanced requirements for skin blood flow. Although such requirements were certainly smallest at 25°C water temperature, heart rate at this temperature was slightly higher than at 30°C suggesting reflex activation of sympathetic control by cold signals from the skin. There was a significantly greater increase in mean skin and rectal temperatures in subjects exercising on the treadmill in air, compared to those exercising in water at 25°C. Accepted: 22 May 1998